This invention relates to methods and apparatus for thermocontact welding of thermoplastic sheets and the product produced thereby, and more particularly to a thermocontact welding method and apparatus for producing welded seams joining polyolefin sheets (such as polypropylene or polyethylene) and the welded product produced thereby.
The invention finds particular application in the fabrication of such articles as album pages or leaves for storing and displaying cards depicting persons of common professional interest, such as sports figures, as well as album pages or leaves for storing and displaying photographs. Such articles are commonly fabricated of sheets of polyvinyl chloride, or vinyl, whereby linear seams are produced for welding such sheets in a configuration for producing pockets for inserting such cards or photographs, usually by radio-frequency welding techniques.
Polypropylene has certain advantages over vinyl as a sheet material for fabricating such articles Polypropylene is substantially chemically inert and will generally not react with dyes and other surface components of sports cards and photographs, and is physically resistant to extreme heat and cold. Polypropylene is highly suitable for archival applications, and polypropylene leaves having pockets for containing cards or photographs are particularly suitable for the long term storage of such contents.
However, unlike vinyl, attempts to weld polypropylene sheets (as well as other polyolefin sheets) by radio-frequency welding techniques have been in general unsatisfactory. Instead, thermocontact welding is generally employed, although attempts to produce a solid weld seam by thermocontact welding have previously caused the welded sheets to exhibit a tendency to curl or otherwise deform, thought to be a result of polypropylene's sensitivity to heat. In order to prevent curling or deformation, prior art thermocontact methods for welding polypropylene sheets have utilized discontinuous or intermittent die surfaces for producing discontinuous or intermittent welded seams--i e. the welded seam is comprised of a sequence or series of welded dots or short dashes with unwelded material between successive dots or dashes. As used herein, the term "solid" as applied to a welded seam is meant to describe a non-intermittent seam, and as applied to a die surface is meant to describe a non-intermittent die surface.
In prior art thermocontact welding, two superposed thermoplastic webs are suspended beneath a downwardly facing upper die which is configured with seam forming die surfaces. Both the upper die and the lower platen having upwardly facing die surfaces are heated, with the temperature of the lower platen and die surfaces being higher than the temperature of the upper die surfaces. When thermocontact welding is utilized for welding polypropylene sheets by means of intermittent welds, the lower platen and die surfaces are conventionally at a temperature higher than the melting point of polypropylene, while the upper die surfaces are conventionally at a temperature lower than the melting point of polypropylene.